1. Field of the Invention
The present invention relates to the improvements of an intake-air quantity control apparatus for an internal combustion engine equipped with a variable valve timing system capable of electronically arbitrarily controlling an intake- and/or exhaust-valve timing, depending on operating condition of the engine/vehicle operating conditions.
2. Description of the Prior Art
In recent years, there have been proposed and developed various electronically-controlled variable valve timing systems which are capable of operating intake and exhaust valves electromagnetically or hydraulically. One such electronically-controlled variable valve timing system having electromagnetically-operated valve units has been disclosed in Japanese Patent Provisional Publication No. 8-200025. In the Japanese Patent Provisional Publication No. 8-200025, intake and exhaust valves are electronically arbitrarily controlled by means of an electromagnetic-solenoid valve controller, in place of a typical cam-drive mechanism. Japanese Patent Provisional Publication No. 7-317516 has disclosed an electronically-controlled, hydraulically-operated variable valve timing system having a pair of slidable plungers defining a pressure chamber therebetween, and an electromagnetic spill valve provided for each engine cylinder and fluidly connected to the pressure chamber for the purpose of working-fluid supply and working-fluid cut-off to the pressure chamber. The upper plunger is permanently pressed on the cam profile of a cam for producing reciprocating motion of the upper plunger during rotation of the cam, whereas the lower plunger is in abutted-engagement with the upper end of the valve stem. Thus, the motion of the cam is transmitted from the upper plunger via the working oil fed into the pressure chamber to the lower plunger. The working fluid in the pressure chamber can be exhausted therefrom via a flow-restricting orifice to cause a dampening effect. An intake valve closing point or an intake valve close timing (often abbreviated to "IVC"), an intake valve open timing point (often abbreviated to "IVO"), an exhaust valve closure timing (often abbreviated to "EVC"), and/or an exhaust valve open timing (often abbreviated to "EVO") can be adjusted by electronically controlling the activation and de-activation of the spill valve for each engine cylinder.